It has been known in the prior art to prepare phosphates of metals of Group II of the Periodic Table, usable as catalysts for selective cleavage of &gt;C--O-bonds in organic compounds, and, specifically, for converting 4,4-dimethyl-1,3-dioxane into isoprene, as well as for dehydration of alcohols, by precipitating tertiary phosphates of Group II metals from aqueous solutions of their salts taken in conjunction with water-soluble salts of phosphoric acid, followed by separating the precipitate, washing the paste obtained, and shaping it into catalyst granules (cf U.S. Pat. No. 3,872,216).
Catalysts prepared by the above technique, however, are characterized by low selectivity, equal to 78 to 82 mole %, low activity resulting in DMD space velocities as low as 0.7 h.sup.-1, and high operating temperatures equal to about 375.degree. C.
Selectivity is defined herein as the ratio of the amount in moles of isoprene formed to the amount in moles of DMD converted. Selectivity is expressed in percent.
Selectivity is quantitatively depedent upon catalyst composition and structure, as well as upon the process conditions under which the catalyst operates.
Increased selectivity will lead to reduced DMD consumption rates per unit of finished product. The relatively low selectivity of the catalyst obtainable by the aforesaid prior-art technique would result in high feedstock consumption rates in isoprene production, varying between ca. 2.10 and 2.25 kg of DMD per 1 kg of isoprene.
The activity of catalysts is dependent upon their acidity which is determined by the number and efficiency of the active centres and can be characterized by the DMD conversion degree.
DMD conversion degree is defined herein as the ratio of the amounts of DMD converted to DMD used, expressed in percent.
Known in the art is a method of preparing calcium phosphate catalysts, comprising the steps of reacting calcium salts with phosphoric acid salts in aqueous ammonia, followed by separating the resulting precipitate from the reaction mixture, washing, drying and heat treating it with the use of superheated steam or a mixture of steam and air at high temperatures (cf U.S. Pat. No. 3,846,338).
The catalysts obtained by this prior art technique are relatively low in activity.
Furthermore, calcium phosphate catalysts prepared by said technique are lacking in efficiency, which is as low as 0.3 to 0.4 ton/h of iosprene per cubic meter of catalyst.
The efficiency of a catalyst depends on its activity and selectivity, as well as on the feedstock space velocity.
In the prior-art technique, heat treatment of the catalyst is carried out at high temperatures, which involves overheating of the heat carrier to temperatures as high as 650.degree. to 800.degree. C. and high process power inputs, as well as the use of special heat-resistant materials for the reactors adapted to produce the catalyst.
Furthermore, the catalyst obtained by the aforesaid technique has a relatively short useful life of 250 hours.
The catalyst life depends on many factors including catalyst composition and structure, catalyst activity, operating temperatures, and coke deposition. Coke deposition is understood to denote coke depositing on the catalyst in the process of DMD decomposition. It is determinable as the ratio of the amount in moles of coke deposited to the amount in moles of DMD converted, expressed in percent.
In spite of the advantages inherent in the prior-art techniques for preparation of calcium phosphate catalysts, no commercial process for converting DMD into isoprene based thereon has been developed so far, since there is no catalyst as yet with selectivity and stability such as to permit a commercial process with a high yield of the desired product.